Energy ChangesAQA GCSE Chemistry: Topic test

20 questions, 54 marks

AQA GCSE Chemistry

Energy Changes topic test

Total 54 marks

Name

Class

Date

1
A first-aid supplier sells reusable heat packs for treating muscle strains. Each pack contains a clear, supersaturated solution of sodium acetate with a small metal disc inside. Flexing the disc triggers the solution to crystallise rapidly, and the pack becomes warm to the touch within seconds. The pack can be reset by boiling it in water, which redissolves the crystals ready for reuse.
(a)
What term describes a reaction, such as the crystallisation in the heat pack, that transfers energy to the surroundings so their temperature increases?
[1 mark]
AExothermic
BEndothermic
CReversible
DCatalysed
(b)
The pack is reset by boiling it in water until the crystals dissolve back into solution. Compared with the crystallisation reaction, what can be said about this dissolving process?
[1 mark]
AIt is also exothermic
BIt transfers energy from the surroundings to the pack (endothermic)
CIt has no energy change
DIt only occurs at room temperature
(c)
Using ideas about energy transfer, explain why the temperature of the skin next to the heat pack rises when the disc is flexed.
[2 marks]
Total for question 1: 4 marks
2
A fertiliser plant chemist is checking the energy change of the Haber process reaction, nitrogen + hydrogen ⇌ ammonia, using bond energy data. She is given the following average bond energies in kJ per mole of bonds: N≡N = 945, H–H = 436, N–H = 391.
(a)
In the forward reaction N2 + 3H2 -> 2NH3, which bonds must be broken in the reactants?
[1 mark]
ATwo N≡N bonds and one H–H bond
BSix N–H bonds
COne N≡N bond and three H–H bonds
DOne N≡N bond and six H–H bonds
(b)
How many N–H bonds are formed when the reaction produces 2NH3?
[1 mark]
A2
B3
C8
D6
(c)
Calculate the energy needed to break the bonds in the reactants and the energy released forming the bonds in the products, and use your answer to state whether the forward reaction is exothermic or endothermic.
[2 marks]
Total for question 2: 4 marks
3
A marine engineering company is testing a simple cell to monitor corrosion on an underwater pipeline. They dip a zinc rod and a copper rod into seawater (which acts as the electrolyte) and connect them with a wire and a voltmeter.
(a)
Explain why a voltage is produced when the zinc and copper rods are connected in the seawater, referring to the reactivity of the two metals.
[3 marks]
(b)
The engineers replace the copper rod with a magnesium rod, keeping the zinc rod and seawater the same. Predict and explain the effect on the voltage produced, and suggest one practical reason why this new pair of metals might not be reliable for long-term corrosion monitoring.
[4 marks]
Total for question 3: 7 marks
4
A drone manufacturer is designing a long-range delivery drone and is comparing two ways of using hydrogen gas as an energy source: burning it directly in a small combustion engine, or using it in a hydrogen fuel cell to generate electricity for an electric motor. In both cases the overall reaction is the same: hydrogen reacts with oxygen from the air to produce water.
(a)
Using the idea of bond energies, explain why the combustion of hydrogen (2H2 + O2 -> 2H2O) is an exothermic reaction, and describe how a reaction profile diagram would represent this reaction in terms of the relative energies of reactants and products and the activation energy. You are not required to calculate an exact energy value.
[6 marks]
(b)
Explain how a hydrogen fuel cell produces electricity from hydrogen and oxygen, and evaluate, for the drone manufacturer, the advantages and disadvantages of using a hydrogen fuel cell rather than burning hydrogen directly in a combustion engine.
[6 marks]
Total for question 4: 12 marks
5
A drinks company sells a self-cooling can. The base of the can contains a separate compartment with solid citric acid and solid sodium hydrogencarbonate. When the consumer presses a button, a seal breaks and water from the drink mixes with the two solids, and they react together. Within a few minutes, the drink inside the can feels noticeably colder.
(a)
What term describes a reaction, such as the one in the can's base compartment, that takes in energy from the surroundings so their temperature decreases?
[1 mark]
AEndothermic
BExothermic
CCombustive
DDisplacement
(b)
Which observation from the scenario provides the best evidence that the reaction in the can is endothermic?
[1 mark]
AThe reaction only happens when a button is pressed
BThe temperature of the drink decreases after the reaction starts
CThe reaction involves two solids and water
DThe reaction takes a few minutes to complete
(c)
Explain, in terms of energy transfer, why the drink becomes colder once the citric acid and sodium hydrogencarbonate react.
[2 marks]
Total for question 5: 4 marks
6
An engineer investigating vehicle emissions is calculating the energy change when nitrogen and oxygen in the air react inside a hot engine to form nitrogen monoxide: N2 + O2 -> 2NO. She uses these average bond energies, in kJ per mole of bonds: N≡N = 945, O=O = 498, N=O (in NO) = 630.
(a)
Which bonds are broken in the reactants of this reaction?
[1 mark]
AOne N=O bond
BTwo N=O bonds
COne N≡N bond and one O=O bond
DOne N≡N bond only
(b)
How much energy, in kJ, is needed to break all the bonds in one mole of N2 and one mole of O2?
[1 mark]
A1260
B630
C2205
D1443
(c)
Calculate the energy released when the bonds in the products (2NO) form, and use this with your answer to part (b) to explain why this reaction is endothermic, and why it is normally only significant at high engine temperatures.
[2 marks]
Total for question 6: 4 marks
7
A small ferry operator on a Scottish island is deciding between two zero-emission power systems for a new passenger ferry: a bank of rechargeable lithium-ion batteries, or a hydrogen fuel cell system. The ferry makes several short crossings each day and returns to the same dock each evening.
(a)
State what is meant by a rechargeable cell, and explain how it differs from a non-rechargeable cell such as those used in some torches.
[3 marks]
(b)
Evaluate, for this ferry route, whether a rechargeable battery bank or a hydrogen fuel cell system would be more suitable, referring to how each is 'refuelled' and to the length and pattern of the ferry's daily crossings.
[4 marks]
Total for question 7: 7 marks
8
A camping equipment company is testing a portable stove that burns propane gas (C3H8) for cooking. Propane needs a spark from an igniter to start burning, but once alight, the flame continues to burn steadily, releasing a large amount of heat to warm the pan above it. Complete combustion of propane produces carbon dioxide and water: C3H8 + 5O2 -> 3CO2 + 4H2O.
(a)
Explain, in terms of activation energy and energy transfer, why the stove's igniter is needed to start the reaction even though the overall combustion of propane is strongly exothermic, and describe how this would be shown on a reaction profile diagram.
[6 marks]
(b)
Using the bond energies given, calculate the overall energy change for the complete combustion of one mole of propane, and use your answer to explain how this supports the description of the stove flame as releasing 'a large amount of heat'. Average bond energies (kJ per mole of bonds): C–H = 413, C–C = 347, O=O = 498, C=O = 805, O–H = 463.
[6 marks]
Total for question 8: 12 marks

End of questions